The present invention relates to ophthalmoscopes and, more particularly, to binocular indirect ophthalmoscopes for observing and examining the fundus oculi of human eyes.
Binocular indirect ophthalmoscopy offers several advantages over direct ophthalmoscopy, including stereopsis and a much enlarged field of view and depth of field. However, the usefulness and flexibility of binocular indirect ophthalmoscopy has been generally restricted to use in the examination of eyes with substantially clear opacity. Other techniques using infrared scanning laser ophthalmoscopes or infrared fundus cameras are generally employed to examine eyes with media opacity.
For example, scanning laser ophthalmoscopy (SLO) using infrared illumination is preferably used to examine eyes with nuclear sclerotic cataracts or mild vitreous hemorrhage, and in combination with indocyanine green angiography (ICG) to examine the choroidal layer, lesions and subretinal membranes and scars of the eye. SLO uses a low power, focused laser beam, typically Helium-Neon (xe2x80x9cHexe2x80x94Nexe2x80x9d), to scan the size of the aperture through which the reflected radiation is collected. The advantages of SLOs as well as those of infrared fundus cameras are well known to those skilled in the art. Unfortunately, SLOs as well as infrared fundus cameras are prohibitively costly for most clinicians and lack the accustomed stereopsis, which is helpful in evaluating the topography of the fundus features. For these reasons, a need has arisen for an improved binocular indirect ophthalmoscope having comparable capabilities to those of SLOs or infrared fundus cameras, but yet which is relatively inexpensive.
An improved binocular indirect ophthalmoscope for observing and examining the fundus of the human eye is realized by integrating an electro-optic imaging system in the viewing optics of the ophthalmoscope. Advantageously, this permits the direct stereoscopic observation of the fundus using illumination ranging from the near ultraviolet to the infrared, including the visible spectrum.
In accordance with the principles of the present invention, a radiation source illuminates a desired portion of a patient""s fundus, with the reflected radiation brought to focus to produce an aerial image of the fundus. A pair of ophthalmoscope lenses then magnify and image along two different optical paths the aerial image onto separate imaging sensors, such as charge coupled devices (CCDs) or camera tubes, or onto image tubes such as image intensifiers, and the like. For CCDs and camera tubes, visible displays, such as liquid crystal displays (LCDs) or cathode ray tubes (CRTs), then photo-electrically convert the fundus images formed on the imaging sensors and direct corresponding visible images thereof to an observer""s pupils by means of ocular lenses. Of course for image tubes, the observed visible display results when electrons emitted by the photosensitive surface of the image tube strike its fluorescent screen which reproduces the fundus image focused on the photosensitive surface.
Although not limited to, both visible and infrared imaging are readily available by judiciously selecting the spectral characteristics of the radiation source, filters and imaging sensors of the present ophthalmoscope. This is a particularly distinct advantage over prior art binocular indirect ophthalmoscopes which can only operate in the visible spectrum.
The electro-optical indirect ophthalmoscope of the invention is well suited for performing retinal angiography and for assessment of retinal circulation. One practice of the invention places filters in the excitation and detection paths of the ophthalmoscope to perform fluorescein or infrared angiography. For example, excitation of a fluorescein dye, applied to a subject""s eye, in the blue portion of the electromagnetic spectrum elicits fluorescence from the eye in the green portion of the spectrum. Thus, placement of appropriate filters in the excitation and detection paths allows performing such a fluorescein angiography. An alternative practice of the invention excites an indocyanine green dye by near infrared radiation, and detects the fluorescence of the dye at a slightly longer wavelength than that of the infrared excitation, e.g., at 810 nanometers.
Another feature of the ophthalmoscope of the invention is its capability to perform angiography of the peripheral retina of a subject. In particular, the present phthalmoscope can provide viewing of the far periphery of the subject""s retina to document vascular changes. In contrast, a conventional fundus camera provides much more limited range for the examination of the subject""s retina, typically up to approximately 50 degrees across the periphery of the retina. Thus, the present ophthalmoscope can detect retinal lesions that may be missed by a fundus camera.
A further aspect of the indirect ophthalmoscope of the invention relates to the projection of images, shapes, and/or letters onto the retina of a subject. One practice of the invention forms such images on the subject""s retina by projecting light onto the subject""s retina after passing the light through an image former, such as a sliver glass having an imprint of the image thereon. In some preferred embodiments, the source of light for projecting an image onto the subject""s retina is, for example, a laser such as a Hexe2x80x94Ne laser having a power output that is safe for the eye. A variety of different images such as those of the Snellen visual acuity chart, or cartoon images can be employed. The projection of an image onto the subject""s retina provides a fixation target for the subject to follow as an examiner moves the image to observe various portions of the retina. The fixation target in conjunction with other elements of the ophthalmoscope of the invention allows detailed examination of the periphery of the subject""s retina. In particular, moving the target image to extreme ranges of the subject""s gaze allows examination of the peripheral retina.
The fixation target allows, among other applications, estimation of the subject""s visual acuity by projection of Snellen letters onto the subject""s retina, assessment of macular function and the pattern of foveal fixation on target, and the study of the preferred extra-foveal retinal locus of fixation in subjects with macular diseases such an age-related macular degeneration. In addition, the fixation target allows mapping of the center of fixation, which is useful for example in preparation for laser photocoagulation of the retina near the center of macula in conjunction with retinal angiography performed with the ophthalmoscope of the invention. The presence of a fixation target in the indirect ophthalmoscope of the invention is particularly useful in pediatric ophthalmology. Children are typically photophobic, and hence are difficult to examine. The fixation target allows projecting an image, for example a cartoon image, onto the child""s retina to assess the behavior of the fixation and macular function.
According to another aspect of the invention, a computer collects the data corresponding to the images of the subject""s retina obtained through the practice of the invention. A software program allows selected manipulation of the collected data. For instance, digital images of the retina can be enhanced according to known digital processing methods. Alternatively, different retinal images can be added together or subtracted from each other to obtain structural information and/or enhance visualization of certain features of the retina.
One aspect of the invention relates to obtaining images of the same portion of a subject""s retina by employing radiation in different regions of the electromagnetic spectrum. For example, four images can be obtained of the same retinal area by employing light in the blue, green, yellow, and the red regions of the spectrum. The images can be digitized and the digital data transferred to a software program. The program can, for example, obtain the difference between the intensity of the blue image and the red image to provide information regarding the subject""s retinal thickness. Alternatively, the difference between the blue and the green images provides information regarding the subject""s inner retinal thickness. In an alternative practice of the invention, the four images with different wavelengths, can be displayed in rapid succession on a monitor to provide the examiner with a three-dimensional image that approximately correlates with the retinal topography.
These and other features of the invention are more fully set forth below with reference to the detailed description of the invention, and the accompanying figures.